Thanks for all the responses!
I've uploaded a picture of my girl as my avatar. It is from about six years ago, when she was at her heaviest (about 45 lbs - a far cry from the skinny 34.5 lbs when we got her!). Even then she was by no means overweight. Regarding the amount of food we are giving her, we have always kept it available to her since the summer when we noticed her weight loss, especially because we couldn't give her treats anymore.
Regarding other reasons for her weight loss, we had a whole set of geriatric blood and urine tests done a couple of months ago because of the urinary incontinence, and they didn't reveal any problems.
The only reason that bloat even crossed my mind is because I read that it is more common in dogs with digestive problems, plus the fact that her seizures a few years ago were determined to be a result of painful gas. That coupled with her chest "appearing" rounded got me worried! It appears, like a couple of you have stated, that bloat occurs within hours, but I have also read about chronic bloat, so that's why I was concerned.
Having said all that, we have made her an appointment for tomorrow. That way we can get her annual checkup done and over with, and we can address our concerns about her weight.